PERT on Critical path question

Submitted by apmeena on Tue, 08/23/2011 - 19:29

Does anyone know how to calculate the below. 


Question:


Together with your team you applied three point estimation on a  Critical path containing two activities.


The following duration uncertainities are calculated using +/-3sigma confidence interval.


The duration uncertaining-defined as Pessimistic minus optimistic estimate - of the first activity is 18 days; the second estimate has an uncertainity of 24 days. Applying the PERT formula for paths, what is the duration uncertaining of the entire path?


A. 21 days


B 30 days


C 42 days


D No statement is possible from information given.


Answer is B. 

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apmeena

Thu, 08/25/2011 - 10:41

Can anyone help on the above calculation?

apmeena

Thu, 08/25/2011 - 12:35

I found it finally. 


For the benefit of all :


The first activity has a sigma of 3 days (18/6), the second activity has a sigma of 4 days (24/6), so combining them the overall sigma is 5 days (SQROOT (3squared + 4 squared)). Therefore 6 sigma of the combination is 30 days (ie 6*5days).

Aditya Mishra

Sat, 08/27/2011 - 23:00

I find your calculation a bit strange. From what I have studied from Rita ( I am yet to take my test on 30th), it should be done like below;

Uncertainity for Activity A  = Standard Deviation = 18

Variance for ACtivity A = 18*18 = 324

Uncertainity for Activity B  = Standard Deviation = 24

Variance for ACtivity B = 24*24 = 576

Total Variance for all activities on CP = 324+576 = 900

SD (Uncertainity) for the entire path = Square Root(900) = 30.

 

Can you tell me where did you get the formula for sigma calculation.

 

Adi

fatraif

Sat, 03/24/2012 - 15:22

http://www.youtube.com/watch?v=vW6CAbP43cI

after calculating the 5 with +/- 3SD  which means 3x5 on right of ya xis of the bill shape diagram and 3x5 on the left side will give 6x5 or 15+15   == 30